This invention relates to a high pressure metal vapor discharge lamp, and more particularly to a starter circuit for the discharge lamp.
In order to start a high pressure metal vapor discharge lamp such as a high pressure sodium lamp, it is necessary to apply a high discharge start voltage to the lamp. For this purpose, a variety of discharge lamp starting units have been proposed in the art.
One example of a conventional discharge lamp starting unit will be described with reference to FIG. 1.
As shown in FIG. 1, a power source 10 is connected in series to an arc tube 12, which is shunted by a series circuit of a thermal switch 14 and a non-linear ceramic capacitor or a ferro electric capacitor 16 (hereinafter referred to as "an FEC 16"). The arc tube 12, the thermal switch 14, and the FEC 16 are built in a bulb 20. The thermal switch 14 is kept closed at room temperature; that is, it is opened when the ambient temperature increases to a predetermined value.
In turning on the lamp, power is supplied to the FEC 16 through the thermal switch 14 which is closed at room temperature, so that the FEC 16 is charged; that is, the latter FEC 16 produces a pulse voltage which induces discharges in the arc tube 12.
As a result, the arc tubes 12 turns on, to increase the ambient temperature. Hence, the thermal switch is opened (or turned off), and accordingly, the generation of the pulse voltage by the FEC 16 is ceased to disconnect a load applied to the FEC 16.
A second example of the conventional discharge lamp starting unit is as shown in FIG. 2. As is apparent from comparison of FIGS. 1 and 2, the discharge lamp starting unit shown in FIG. 2 can be obtained by connecting a semiconductor switch 18 in series to the series circuit of the thermal switch 14 and the FEC 16 in the first example of the conventional discharge lamp starting unit shown in FIG. 1. With the semiconductor switch 18 connected to the series circuit, the pulse voltage generated by the FEC 16 can be used for starting the discharge lamp 12 more effectively.
A third example of the conventional discharge lamp starting unit is as shown in FIG. 3. In the third example, a power source 10 is connected through a ballast 19 to an arc tube 12 (comprising a polycrystalline alumina tube), which is shunted by a series circuit of a thermal switch 14 such as a bimetal switch and an FEC 16. The thermal switch is held closed at room temperature; that is, it is opened when the ambient temperature increases to a certain value.
The discharge lamp starting unit further comprises a start assisting conductor 48 which is extended from the connecting point of the thermal switch 14 and the FEC 16 substantially over the whole length of the arc tube 12 and held in contact with the outer surface of the arc tube 12. That is, the conductor 48 has one end 48a which is a free end, and the other end 48b which is connected to the connecting point of the thermal switch 14 and the FEC 16. Those elements 12, 14, 16 and 48 are built in an outer bulb 20.
In order to turn on the lamp, current is drawn from the power source 10 through the thermal switch 14, which is closed at room temperature, to the FEC 16 to charge the latter 16. As a result, the FEC 16 generates a high pulse voltage. The high pulse voltage together with the supply voltage is applied to the arc tube 12 to induce discharges in the latter 12.
As a result, the arc tube 12 is turned on, and the ambient temperatures is increased, so that the thermal switch 14 is turned off, whereby the oscillation of the FEC 16 is ceased. Thus, the lamp is kept turned on in the ordinary manner.
The start assisting conductor 48 laid over the arc tube 12 is used to apply electric field to the inside of the arc tube to decrease the starting voltage, thereby to enhance the induction of discharges in the arc tube 12 at the start of the lamp.
FIG. 4 shows a fourth example of the conventional discharge lamp starting unit. The fourth example of the conventional discharge lamp starting unit can be obtained by modifying the above-described third example (FIG. 3) as follows: The thermal switch 14 is removed from the third example (FIG. 3), and instead thermal switches 14a and 14b are connected to both ends of the start assisting conductor 48 as shown in FIG. 4. The start assisting conductor 48 is electrically disconnected from the circuit after the lamp has started. The thermal switch 14a connected to one end of the start assisting conductor 48 is connected in series to the power source 10 through the ballast 19. The thermal switch 14b connected to the other end of the start assisting conductor 48 is connected to the FEC 16, and it is closed at room temperature. Therefore, when the lamp is kept turned on, the ambient temperature is increased, whereby the thermal switches 14a and 14b are turned off. As a result, the start assisting conductor 48 is electrically disconnected from the circuit. The fourth example of the conventional discharge lamp starting unit further comprise a semiconductor switch 18 which is connected in series to the FEC 16 which is connected to the thermal switch 14b as was described above; and a resistor 24 which is connected in parallel to the semiconductor switch 18. The resistor 24 is used to stabilize the switching phase.
The above-mentioned examples of the conventional discharge lamp starting unit shown in FIGS. 1 through 4 are disadvantageous in the following points:
The FEC 16 is a ferro-electric ceramic capacitor, which shows ferro-electricity at temperatures lower than the Curie temperature and paraelectricity at temperatures higher.
When the lamp is started, the FEC 16 is at room temperature lower than the Curie temperature. Therefore, the FEC 16 shows ferro-electricity, thus being able to generate the pulse voltage; however, it should be noted that, with the voltage, the FEC 16 is subjected to poling.
FIG. 5 shows the dielectric constant characteristic of the FEC. As is apparent from FIG. 5, the FEC is a ferro-electric element at temperatures lower than the Curie temperature (about 90.degree. C.), the FEC, being a ferro-electric element, is subjected to poling with the pulse voltage generated when the lamp is started.
On the other hand, while the lamp is kept stably operated, the FEC 16 is held at temperatures higher than the Curie temperature by the heat of the arc tube 12, thus becoming a paraelectric element.
When the ferro-electric element subjected to poling once is changed into a paraelectric element (by the raise of temperature in this case), depoling occurs. The current flowing in this case is called "pyroelectric current". The pyroelectric current becomes maximum at a temperature slightly lower than the Curie temperature, and that the FEC is therefore subjected to depoling (cf. Ceramic Engineering for Dielectrics, page 13, by Kiyoshi Okazaki, published by Gakkensha).
When, in each of the circuits shown in FIGS. 1 through 4, the lamp is stably operated, and the thermal switch 14 is turned off at a temperature lower than the Curie temperature of the FEC 16 (which is 90.degree. C. as is seen from FIG. 5), the depoling is carried out through the ceramic grain boundaries of the FEC 16, or by the surface discharge (corona discharge) between the two electrodes of the FEC 16.
Thus, whenever the lamp is turned on and off, the process of poling and depoling is carried out. In the case of a high- pressure metal vapor discharge lamp, the outer bulb is highly evacuated, and therefore the depoling through the grain boundary is great, the resistance of the grain boundaries is decreased, and tan .delta. is increased. Thus, in FIG. 6, the end portion Q closing the hysteresis characteristic curve of poling (P) and electric field (E) is gradually widened; that is, the hysteresis transition becomes dull. As a result, the switching characteristic of the FEC 16 is lowered, and the pulse voltage is decreased. Under this condition, finally the starting of the lamp is impossible, and the service life of the lamp may be shortened.
Further, in each of the conventional discharge lamp starting units shown in FIGS. 3 and 4, while the arc tube 12 is being operated, the start assisting conductor 48 is held in contact with the arc tube 12, whereby the wall of the arc tube 12 is partially increased in temperature; that is, the wall of the arc tube 12 becomes non-uniform in temperature distribution, which may crack the wall of the arc tube 12. Furthermore, since surface leaked voltage is applied to the start assisting conductor 48, the sodium in the arc tube 12 may leak through the wall of the arc tube 12.
In the case of the conventional discharge lamp starting unit shown in FIG. 3, while the lamp is operated, some voltage is applied through the FEC 16 to the arc tube 12. Therefore, with a discharge lamp in high operating temperature, the loss of sodium is increased. Accordingly, the starting unit is not applicable to high-power discharge lamps as operating on higher temperature of the arc tube. On the other hand, as the wall of the arc tube 12 is increased in temperature, the insulating resistance of the wall is decreased, so that the arc discharge column in the arc tube 12 is electrically connected to the FEC 16 as if there were a resistor between them. As a result, a high voltage is applied to the FEC 16, so that migration occurs with the silver of the metallized film electrode, whereby the pulse voltage is decreased, and the FEC 16 itself may be deteriorated soon.
In case of the conventional discharge lamp starting unit shown in FIG. 4, although the start assisting conductor 48 is disconnected from the circuit, the arc potential in the arc tube 12 is applied through the arc tube wall to the start assisting conductor 48. Therefore, the discharge lamp starting unit also causes the loss of sodium. In addition, the thermal switches 14a and 14b connected to both ends of the star assisting conductor 48 are not practical in use. That is, in each of the thermal switches, the contact pressure is difficult to adjust. And in the case of a discharge lamp with a small outer bulb, it is rather difficult to install the thermal switch therein, because the outer bulb is not large enough in space.
In order to eliminate these difficulties, the following discharge lamp starting unit has been proposed: That is, in the discharge lamp starting unit as shown in FIG. 3 or 4, a thermally operating piece such as a bimetal element is connected to at least one end of the start assisting conductor, and the free end of the thermal operating piece is fixedly welded to a post. The contact pressure of the thermally operating piece is so adjusted that, while the lamp is operated, the start assisting conductor is moved away from the wall of the arc tube by the heat produced thereby (Japanese Utility Model Application Examined Publication No. Sho. 56-5815).
The start assisting conductor construction as described above is advantageous in that the leakage of the sodium in the arc tube is prevented, and the wall of the outer bulb is scarcely cracked. However, the unit is still disadvantageous in the following points: It is true that the start assisting conductor is held away from the arc tube by means of the thermally operating piece while the lamp is operated; however, when the lamp is started again, after power is interupted for a few seconds and the lamp is turned off, which calls "restart", sometimes the start assisting conductor is brought into contact with the outer wall of the arc tube after the FEC generates the pulse voltage. That is, in this case, the FEC generates the pulse voltage under the condition that the start assisting conductor does not work and the lamp does not light up. Therefore, unavoidably the FEC is deteriorated earlier. On the other hand, the case may be considered in which the start assisting conductor is brought into contact with the outer wall of the arc tube before the FEC generates the pulse voltage. In this case, in order to obtain the pulse voltage which positively starts the discharge lamp or restarts it, the FEC must be at a temperature lower than its Curie point.